This invention relates to apparatus for polishing semiconductor or similar type materials, and more specifically to such apparatus which permits batch processing of the wafers with improved uniformity, throughput and yield.
Polishing an article to produce a surface which is highly reflective and damage free has application in many fields. A particularly good finish is required when polishing an article such as a wafer of semiconductor material in preparation for printing circuits on the wafer by an electron beam-lithographic or photolithographic process. Flatness of the wafer surface on which circuits are to be printed is critical in order to maintain resolution of the lines, which can be as thin as 1 micron or less. The need for a flat wafer surface, and in particular local flatness in discrete areas on the surface, is heightened when stepper lithographic processing is employed.
Flatness is quantified in part by a total thickness variation measurement (TTV) and site total indicated reading (STIR). TTV is the difference between the maximum and minimum thicknesses of the wafer. STIR is the sum of the maximum positive and negative deviations of the surface in a small area of the wafer from a reference plane, referred to as the "focal" plane. Total thickness variation in the wafer is a critical indicator of the quality of the polish of the wafer. Presently, flatness of the polish surfaces of the wafers are not significantly improved and may be worsened by the polishing process. In batch processing, there will be a significant number of wafers which fail to meet flatness and polishing specifications after polishing, thus adversely affecting yield in commercial production.
Conventional polishing machines include an annular polishing pad mounted on a turntable for driven rotation about a vertical axis passing through the center of the pad. The wafers are fixedly mounted on pressure plates above the polishing pad and lowered into polishing engagement with the rotating polishing pad. A polishing slurry, typically including chemical polishing agents and abrasive particles, is applied to the pad.
In order to achieve the degree of polishing needed, a substantial normal force presses the wafers into engagement with the pad. The coefficient of friction between the pad and wafer is quite high, oftentimes in the vicinity of two. These high forces can give rise to certain distortions in the polish, such as by creating a vertical component of the frictional force at the leading edge of a wafer as it encounters an area of particularly high frictional interaction with the polishing pad. A change in the net vertical force applied to the wafer locally changes the polishing pressure and the polishing rate of the wafer, giving rise to distortions in the polish.
Where batch processing is employed, several wafers are rigidly mounted to a single pressure plate. Different regions of the polish face engaging the polishing pad travel along separate paths because the wafers are rigidly attached to the pressure plate. A discontinuity in the pad (e.g., a small lump or an area of glazed slurry) may repeatedly encounter one region of the wafer and not another, causing an imperfection in the polish in the one region. Further, forces and vibrations which are generated by the interaction of one wafer with the polishing pad are transmitted through the rigid structure of the pressure plate to undesirably affect the polishing rate and mechanical characteristics of other wafers on the pressure plate. Moreover, wafers to be polished by batch process must be presorted so that all wafers to be mounted at one time on a single pressure plate are of the same thickness to a high degree of accuracy. Otherwise, the pressure plate is tilted from the horizontal enough to introduce a nonuniform application of pressure to the wafers on the plate, causing undesirable variations in the polish finish between wafers mounted on the same pressure plate and over the polish surface of a single wafer.
The problems of yield associated with batch processing are somewhat alleviated by single wafer processing, in which each wafer has its own pressure plate. Single wafer processing eliminates the problems of forces transmitted through the pressure plate from one wafer to another. However, single wafer polishing has a very low throughput because only a single wafer per pressure plate is polished at a time.